44 BASES AND CRITERIA. 



for subalpine trees, some of which change their form and not their seasonal 

 phenomena, while others reverse this behavior (Engler, 1912 : 3). The 

 response of herbaceous species grown in two or more habitats is equally signifi- 

 cant. Some are so responsive or plastic that both form and structure show 

 practically perfect adjustment to each habitat in the first generation. Others 

 modify the form and not the anatomy, and still others the interior of the leaf 

 but not its form. There are all degrees of completeness of response to the 

 stable plant, in which form and structure change little, and all the adjustment 

 must be secured through function (E. S. Clements, 1905: 93). 



As a consequence, the indicator value of any species can not be known until 

 its functional response has been measured and correlated with the structural. 

 This does not mean that the constant occurrence of a species in certain condi- 

 tions can not be turned to practical account, but it does suggest the wisdom of 

 regarding such correlation as tentative until the functional indication has been 

 determined. The latter will also solve the puzzles presented by communities 

 in which very different life-forms, such as evergreen and deciduous trees, 

 appear to flourish on equal terms. The most striking case of the masking of 

 the real response by habit is seen in such leafless rush-forms as Sairpus lacus- 

 tris and Equisetum, in which it is now proved that the functional response is 

 that of a hydrophyte (Sampson and Allen, 1909:49; Dosdall, 1919). 



Individuality in response. — Indicator values center about the species. Uni- 

 formity of behavior under uniform conditions and clear-cut adjustment when 

 these are changed are the essentials of a good indicator. For these reasons it 

 is important to deal chiefly with species which are represented by many indi- 

 viduals, such as dominants and subdominants, and hence to use the community 

 as the basis for indicators. This makes it necessary to determine the range of 

 individual response in function and growth as well as in structure. In devel- 

 oping the use of standard plants as instruments, this matter is of the first 

 importance. While the question of standardization will always enter, it will 

 be convenient to use those species in which the individuality or functional 

 response is slight. In the use of indicators, the range of individual behavior 

 is a less important consideration than the knowledge of the range. 



Sampson and Allen (1909 : 37) have studied the individual behavior of four 

 montane species as to transpiration and reached the following conclusion : 



"Only slight variations occur, not usually exceeding 3 mg. per square centi- 

 meter for a period of 12 hours. Therefore, it may be concluded that plants of 

 the same species grown in the same habitat when tested under the same 

 physical conditions show but slight variation in transpiration per unit of 

 surface exposed." 



Effect of extreme conditions. — The significance of extreme conditions for 

 response and the relation to indicator values is shown by the case of xerophytes 

 and halophytes. While the latter are now known to be merely xerophytes of a 

 somewhat special type, they were long thought to constitute a distinct class. 

 This is still true in a measure of those species which tolerate salts directly 

 injurious, but it is well known that the majority owe their impress to physio- 

 logical dryness due to the abundance of salts. But, while halophytes are 

 indicators of arid conditions, it is a special type of aridity, and the indication 

 must not be assumed to mean just what it does in ordinary soils. 



